Termination of the hormone-sensitive period for differentiation of the sexually dimorphic nucleus of the preoptic area in male and female rats.

The volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) in the rat brain is several-fold larger in males than in females. The volume of the SDN-POA can be influenced significantly by the hormone milieu during early postnatal life. The purpose of the present study was to identify when termination of the sensitive period occurs during which exogenous androgen administration influences SDN-POA volume in males gonadectomized on the first day of postnatal life (fales) or intact females. Analysis of the SDN-POA in fales showed that testosterone propionate (TP, 500 micrograms) treatment on days 2, 3, 4, or 5, significantly increased its volume over values from oil-treated fales. In contrast, TP treatment in fales on days 6, 7, or 8, failed to increase SDN-POA volume. A similar pattern was observed in females treated with TP. Females treated with TP (500 micrograms) on days 2, 3, 4, or 5, showed a significant increase in SDN-POA volume compared to the values from oil-injected animals, while the same TP treatment in females on days 6, 7, or 8, resulted in no such enhancement. The absolute and relative change in SDN-POA volume following postnatal androgen treatment is greater in males than in females. We conclude that (1) SDN-POA development is sensitive to hormone action through postnatal day 5 and then abruptly becomes insensitive to this dosage of TP, and (2) although the temporal pattern of the response is similar in males and females, androgen exposure postnatally results in a consistently greater increase in the male SDN-POA volume than in the female's. This greater response may be due to exposure prenatally to endogenous androgen in males.